1. Field of the Invention
The present invention lies in the field of tape-shaped silicon crystals and of methods for producing the same wherein a horizontally moving carrier member is drawn over the surface of a silicon melt.
2. Description of the Prior Art
A procedure for making tape-shaped silicon crystals for semiconductor components may be derived from the U.S. Pat. No. 4,305,776. A transportable tape, for example, a graphite net, provided with a special perforation system is employed as the nucleation center carrier and the crystallization of the molten silicon in the meshes of the net is effected by a stream of cooling gas.
Cost-favorable silicon for solar cells can be manufactured in this way since the silicon is directly converted into planar form from the melt without intermediate steps.
In addition to the efficiency of the solar cells made from this planar silicon, the surface velocity with which this material is produced is the main criterion for the economic feasibility of the method. The surface velocity is fundamentally limited by the latent heat released in the crystallization which must be dissipated to the same degree to which it arizes. Due to the favorable relationship of crystallization face to heat-dissipating surface, horizontal methods of silicon crystal formation offer considerable advantages over vertical methods. Silicon tapes can thus be drawn with speeds up to about 80 cm/min with the so-called LASS method (=low angle silicon sheet) disclosed in an article by Bates and Jewett in Proceedings of the Flat-Plate Solar Array Project Research Forum on the High-Speed Growth and Characterization of Crystals for Solar Cells, July 25-27, 1983, Port St. Lucie, Fla. pages 297-307. This method does not employ any carrier member for the coating. However, it has two disadvantages that shall be explained in greater detail with reference to FIG. 1:
1. The crystallization rate at the front edge 3 (as seen in drawing direction d.sub.z, see arrow) of the silicon crystallization face 4 (equal to the guiding edge of the growing silicon tape 2) must be the same as the drawing speed. At high drawing speeds, this can only be achieved by undercooling of the silicon melt 1 and dendrite growth. Due to the irregularities of the layer formation connected therewith, however, dendrite growth means a reduction in the efficiency of the solar cells fabricated from this material.
2. The desired dimensions of the tape (width, thickness) cannot be kept constant over a longer time. The cause of this is the short time span t during which the silicon layer must form and has contact with the melt (dwell time). It derives from the length L of the crystallization face 4 and from the drawing speed v.sub.z at t=L/v.sub.z =2/80=1.5 sec. Due to the finite heat capacity of the melt vessel 5 and the melt 1, random temperature fluctuations in the melt 1 cannot be leveled out in this short time. They fully influence the temperature gradient in the melt 1 which is essential for the layer growth, that is, they lead to geometry changes of the growing silicon tape 2. The stripper for the silicon melt 1 which is attached to the melt vessel 5 is reference 6.